Acute Dissecting Aneurysm of the Ascending Thoracic Aorta Causing Obstruction and Thrombosis of the Right Pulmonary Artery

Abstract

We present the case of a 69-year-old man with a history of hypertension and a recent pelvic fracture who presented with acute chest pain, shortness of breath, and severe hypotension. The history of recent pelvic fracture and the clinical manifestations, including the sudden onset of acute respiratory distress, hypotension, and hypoxemia, indicated pulmonary embolism; however, at surgery the patient was found to have an acute dissection of the ascending aorta with obstruction and thrombosis of the right pulmonary artery. This case emphasizes the need to consider such a diagnosis in patients who have unilateral absence of perfusion to the right lung.

There is no disease more conducive to clinical humility than aneurysms of the aorta.

—Sir William Osler

Type A acute aortic dissection is a serious and life-threatening condition that requires early diagnosis and prompt surgical treatment. Despite remarkable advances in the diagnosis of aortic dissection due to the availability of advanced imaging techniques, the most important factor leading to a successful diagnosis is a high index of suspicion by the examining physician. ¹ Of 24,000 acute dissections of the aorta occurring annually in the United States, only 2,000 are diagnosed antemortem. ² Similarly, pulmonary embolism, which is the 3rd most common cardiovascular condition in the United States and results in as many as 50,000 deaths annually, is poorly diagnosed. ³ Recognized cases of pulmonary embolism constitute only a small percentage of actual episodes, because the diagnosis is elusive and the differential diagnosis wide.

We describe a case of acute type A aortic dissection with an unusual presentation. The onset of the dissection was associated with acute extrinsic compression and consequent thrombosis of the right pulmonary artery. Suspected pulmonary embolism obscured the clinical picture of aortic dissection, leading to a misdiagnosis.

Case Report

A 69-year-old man with a history of hypertension and a recent fracture of the pelvis (15 days earlier) was transferred from an outlying hospital to our institution. One hour previously, he had experienced the sudden onset of acute chest pain, followed, within a few minutes, by shortness of breath and severe hypotension. Intravenous heparin, inotropic support, and mechanical ventilation had been given without any change in his condition. At the time of arrival at our institution, the patient had an elevated jugular venous pressure, peripheral cyanosis, blood pressure of 65/45 mmHg, and an irregular pulse rate of 110 beats/min. The peripheral pulses were weak but symmetrical. Arterial blood gas analysis showed severe hypoxia (PaO₂, 40 mmHg) with compensatory alkalosis (PaCO₂, 26 mmHg; pH, 7.45). A 12-lead electrocardiogram showed S-T segment depression and T wave inversion in leads III, AVF, V₁ and V₃. Chest radiography showed moderate enlargement of the mediastinum. A transthoracic echocardiogram was not of diagnostic quality, because of a poor acoustic window. Laboratory results showed a marked rise in fibrin degradation products (FDP) and in plasmin degradation of cross-linked fibrin (D dimer, 15.2 μg/mL). Urgent pulmonary digital subtraction angiography showed right pulmonary artery thrombosis with total peripheral avascularity (Fig. 1). A catheter introduced into the right branch of the pulmonary artery could not be advanced more than a few millimeters.

Fig. 1 Pulmonary angiogram showing occlusion of the right pulmonary artery and its branches. In the right main pulmonary artery, the reduction of contrast filling is the result of compression of the artery by the dilated ascending aorta.

In view of the patient's poor hemodynamic status and the massive obstruction of the right pulmonary artery, urgent surgical thrombectomy was deemed necessary. After the induction of anesthesia, the chest was opened through a median sternotomy and the pericardium was entered. A moderate hemorrhagic pericardial effusion was present. The ascending aorta, which was markedly dilated, showed a bluish ecchymosis of its wall, a typical feature of acute dissection. The dissection process extended beyond the aortic arch: the right main pulmonary artery, immediately after its origin, was compressed by the dilated aorta.

The ascending aorta was replaced with a 28-mm Dacron graft prosthesis. The distal anastomosis was performed first, with use of an open technique, during a period of 18 minutes of hypothermic circulatory arrest at 20 °C (nasopharyngeal temperature). During the rewarming, a right pulmonary artery thrombectomy was performed, and a large thrombus was removed. Weaning the patient from cardiopulmonary bypass was uncomplicated, and perfusion was discontinued after 111 minutes of pump time.

Postoperatively, there was an immediate marked rise in the arterial PaO₂ level. Color-coded Doppler echocardiography excluded peripheral emboli. A ventilation-perfusion quotient (V/Q) lung scan confirmed multiple V/Q peripheral mismatched defects in the right lung, compatible with distal residual obstruction of the right pulmonary artery branch.

The patient was discharged from the intensive care unit after 72 hours, left the hospital 12 days later, and is doing well 14 months after the operation.

Discussion

Occlusion of the pulmonary artery by an acute aortic dissecting aneurysm is an extremely rare event: only a few cases have been reported in the English-language medical literature since the 1st description by Buja and colleagues in 1972. ^4–8 They reported an autopsy case of acute dissection of the ascending aorta with a hematoma compressing the right pulmonary artery and illustrated how the anatomic relationship of the aorta and pulmonary artery and their common tunica adventitia made the right pulmonary artery susceptible to extrinsic compression after aortic dissection.

Compression of the right pulmonary artery can produce varying degrees of obstruction, with clinical manifestations ranging from unilateral pulmonary edema ⁸ to frank right pulmonary artery occlusion. ⁶ In our patient, the clinical manifestations, including the sudden onset of acute respiratory distress, hypotension, and hypoxemia, strongly suggested pulmonary embolism. The final findings accurately reflected the clinical events, although from a pathologic point of view, the pulmonary artery obstruction was due to the initial compression of the pulmonary artery by the dissection, and its subsequent complete occlusion was due to locally induced thrombus and not to an embolism.

The acute presentation, the poor clinical status of the patient, and the complete right pulmonary occlusion requiring urgent surgical thrombectomy precluded any consideration of thrombolytic therapy, which could have been catastrophic had it been administered.

The clinical history of a recent fracture of the pelvis, as well as the mode of presentation, misled the physicians from the correct diagnosis. Preoperative computed tomographic scanning of the chest or magnetic resonance imaging would most likely have revealed the dissection; however, the urgency of the situation did not allow time for such investigation. This case emphasizes the importance of including acute dissection of the ascending aorta in the differential diagnosis of patients who have unilateral absence of perfusion to the right lung.